A restaurant is a place — usually inside a building — where you go to eat food, which, most of the time, you must pay for.
Restaurant comes from the French restaurer, which means "to provide food for." Restaurants will provide food for you if you've got the cash, and most of the time they'll even let you sit down right there and eat it. Common examples of restaurants include burger joints, cafeterias, pizzerias, sandwich shops, steak houses, seafood shacks, (some) hot-dog stands, ice cream parlors, taquerias, Chinese takeout, (some) bakeries, and fine-dining establishments.